Photomultiplier detector protection device and method

ABSTRACT

A photomultiplier detector protection system and method utilizing a negative feedback loop to maintain the photomultiplier detector output below a predetermined output. The feedback loop comprises a comparator responsive to the photomultiplier detector output and also responsive to a predetermined limit signal. The comparator output is applied to a summing amplifier which also receives a voltage adjust signal. The output of the summing amplifier controls the output of a power supply which is in turn applied to the photomultiplier detector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to photomultiplier detectorsand, more particularly, to a protection device and method for usetherewith. The invention is particularly suited for protecting aphotomultiplier detector against damage which can result from theapplication of excessive incident light flux while maintaining theoperation of the detector at a safe level until the excessive incidentlight flux is reduced or removed.

2. Description of the Prior Art

Photomultiplier detectors are used in numerous applications where lowlight levels are sensed or detected. In such detectors, light energy isdirected onto a photo-emissive cathode which emits electrons inproportion to the incident light energy. The emitted electrons aredirected at a first of a series of dynodes that emits an increasednumber of electrons through a process known as secondary emission. Theincreased number of secondary emission electrons from each dynode iscascaded onto the next dynode within the series, producing an outputcurrent through an anode positioned to receive the electron emissionfrom the last dynode. The amplification resulting from the secondaryemission at each dynode can result in extremely sensitivephotomultiplier detectors having gains or ratios of anode currents tocathode currents as high as 10⁸ or more.

Each dynode within the dynode series is connected to a source ofrelatively high voltage, the voltage increasing from the first to thelast dynode in the series. Generally, a high voltage power supply servesas a high voltage source for a voltage divider network of fixedresistors that in turn supplies the various voltages required by thedynode series. The output current flowing from the photomultiplierdetector is proportional both to the incident light energy falling uponthe photo-emissive cathode and also to the voltage applied to thevoltage divider network and in turn to the dynodes. The voltages appliedto the dynodes from the voltage divider network are sometimes referredto hereinafter collectively as "dynode voltage". Thus for a givenconstant incident light flux or intensity, the output current flowingfrom the detector anode can be varied by varying the dynode voltage(that is, by varying the high voltage applied to the voltage dividernetwork). Since the sensitivity of the photomultiplier detector can beexpressed as the output current flow for a given constant incident lightflux, it is seen that the detector sensitivity is varied by varying thedynode voltage.

A problem associated with using photomultiplier detectors is protectinga detector from combinations of incident light flux and dynode voltagewhich can produce a destructively high current flow within the detector.Such a situation can occur where the dynode voltage and thus thedetector sensitivity is initially set for a first relatively lowincident light flux. However, if the incident light flux is increased,the current multiplying effect within the photomultiplier device canraise the current flowing therethrough to a level which can damage ordestroy the detector. Hence, it is necessary to protect thephotomultiplier detector from combinations of incident light and dynodevoltage which can result in an excessive current flow through thedetector.

The problem of photomultiplier detector protection arises in the fieldof spectrophotometry where photomultiplier detectors are widely used inspectrophotometers to sense the light passing through a sampleundergoing spectral analysis.

Several situations occur where the photomultiplier detector can beexposed to light intensities which can result in damage to the detector.Generally, spectrophotometers have a sample compartment that receives asample for analysis. When placing the sample into the samplecompartment, the ambient room light can flood the compartment and canenter the optical path leading to the photomultiplier detector. Ambientlight levels in such an instance can be considerably above the lightlevel applied to the detector during normal operation. Also, changes inlight wavelength can result in substantial variations in incident lightintensity because of the wavelength-dependent differences in spectralenergy content of light emanating from the source within thespectrophotometer and because of non-uniformities in the dispersion oflight by the optical elements within the spectrophotometer's opticalpath. Variations in photomultiplier detector spectral sensitivity forchanging incident light wavelength can also lead to detector damage.Moreover, the photomultiplier detector can be exposed to high lightintensities during servicing of the spectrophotometer instrument as, forexample, when the optical path is opened to room ambient light. In eachof these instances, it is possible for the photomultiplier detector toreceive sufficient incident light flux which, along with the applieddynode high voltage, can result in damage to the detector.

Generally, two types of spectrophotometers are known. The first type isa double-beam spectrophotometer while the second type is a single-beamspectrophotometer.

In a dual-beam spectrophotometer, light from a light source is rapidlyalternated between a sample beam path and a reference beam path. Asample material and a reference material are placed in the respectivepaths and the sample and reference beams are then multiplexed to form acombined single beam that is applied to a photomultiplier detector. Theoutput current from the photomultiplier detector is demodulated toprovide reference and sample signals corresponding to the lightintensities in the respective reference and samp1e paths. The referencesignal is applied to a dynode voltage control circuit which adjusts thedynode voltage to maintain the reference signal at a predetermined levelrelated to a predetermined output current from the detector. Thepredetermined output current is selected to be well within the normaloutput current range of the detector.

If the reference signal varies from the predetermined level, as maynormally result from changes in the light source intensity or drift inthe photomultiplier detector sensitivity, the dynode voltage controlcircuit adjusts the dynode voltage in a direction and by an amountnecessary to correct for the difference between the reference signal andthe predetermined level. The dynode voltage control circuit continuallyoperates in this manner to provide a relatively stable reference signal.As is well known in the art, the sample signal is compared with thereference signal to determine, for example, the transmittance of thesample material as compared to the reference material.

The reference signal adjustment process just described can also serve toprotect the photomultiplier detector from over-current damage. Forexample, if stray ambient light should fall on the detector or if theintensity of the light source should vary considerably, the dynodevoltage control circuit produces a corresponding change in the dynodevoltage and a reduction in the photomultiplier detector sensitivity. Thecontinual adjustment of dynode voltage thus maintains the detectoroutput current within the normal output current range and substantiallyeliminates damage to the detector. In this way, the dynode voltagecontrol circuit provides inherent protection of the photomultiplierdetector in a double-beam spectrophotometer.

However, such inherent protection is not present in a single-beamspectrophotometer instrument. In such an instrument, the sample andreference materials are measured at different times in the same opticalpath. The reference material is first placed into the single-beamoptical path and the dynode voltage is adjusted so that the detectoroutput current is equal to a predetermined reference level within thenormal output current range of the detector. Once the adjustment iscompleted, the dynode voltage is held constant. The reference materialis removed from the single-beam optical path and is replaced with thesample material whereupon sample measurements are made. As is known inthe art, the sample measurements are compared to the reference level todetermine characteristics of the sample such as transmittance. If theintensity or flux of the light incident upon the detector during thesample measurement period increases substantially, the constant dynodevoltage applied during this time can cause a damaging over-currentcondition in the detector. Thus, it is desirable to provide protectionfor the detector in a single-beam spectrophotometer so that widevariations in incident light flux will not lead to damage or destructionof the detector.

One way known for protecting the photomultiplier detector in suchinstruments is to provide a mechanical shutter arrangement which closesthe light path from the sample compartment to the photomultiplierdetector when the compartment is opened. Although this can be asatisfactory solution to the problem during normal operation of thespectrophotometer, the shutter arrangement increases the mechanicalcomplexity of the spectrophotometer. Also, shutters can fail or becomesluggish and cannot be provided to protect the detector against ambientlight in all circumstances such as, for example, when thespectrophotometer is being serviced as described above.

Another form of photomultiplier detector protection known to applicantsis included in a single-beam spectrophotometer instrument designated themodel DU®-8, manufactured and sold by the assignee of the presentapplication. In the DU-8 instrument, a comparing circuit monitors theoutput of the photomultiplier detector during sample measurement periodand compares the output to a predetermined limit indicating that theoutput current is approaching a level above which damage would result.If the output reaches the limit because of excessive incident light fluxfor the constant dynode voltage, the detector dynode voltage is removed.With such an arrangement, however, it was not possible to determine whenthe excessive incident light flux is removed from the detector becausedetector operation ceases with the removal of the dynode voltage.Consequently, once the cause of the excessive light flux condition iscorrected, it is necessary to restart the spectrophotometer operationalcycle, a relatively time consuming and inconvenient process particularlywhere a number of sample measurements are to be taken.

Thus, there is a need for a photomultiplier detector protection circuitwhich not only senses the presence of an excessive light flux conditionin order to protect the photomultiplier detector, but which also is ableto sense when the excessive incident light condition is terminated sothat operation of the spectrophotometer can continue without furtherinterruption.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations described above andmeets the foregoing needs. The present invention resides in a system andmethod which provides substantially continuous control over thephotomultiplier detector sensitivity. The photomultiplier detectoroutput is continuously monitored so that both the beginning and the endof an excessive incident light flux condition can be sensed. Thephotomultiplier detector sensitivity is controlled to prevent detectordamage during periods of excessive incident light flux. When thephotomultiplier detector is used in an instrument such as a single-beamspectrophotometer as described above, operation of the instrument canresume once the excessive incident light flux condition ends withoutfurther interruption. Advantageously, the instrument need not be resetor restarted once an excessive light flux condition is sensed.

To the foregoing ends, the present invention is embodied in a detectorprotection system including a power supply for generating an adjustableoutput voltage in response to a control signal. A photomultiplierdetector is responsive to incident light flux and to the power supplyoutput voltage for generating an output related to the incident lightflux and the magnitude of the power supply output voltage.

A comparator is responsive to a reference signal and is also responsiveto the photomultiplier detector output. The comparator compares thereference signal with the detector output and in turn generates anoutput when the reference signal and the photomultiplier detector outputare in a first predetermined relationship. In a preferred embodiment asdisclosed herein, the reference signal establishes a safe maximum outputlevel for the photomultiplier detector and the comparator generates itsoutput when the photomultiplier detector output is greater than the safemaximum output level.

The system further includes a summing stage responsive to the comparatoroutput and also responsive to an adjustment signal proportional to apredetermined power supply output voltage. When the photomultiplierdetector output is below the limit set by the reference signal, thevoltage of the power supply is set in accordance with the adjustmentsignal applied to the summing amplifier. However, when thephotomultiplier detector reaches the safe maximum output level, thecomparator and the summing amplifier provide a negative feedback loopwhich serves to control the voltage applied to the photomultiplierdetector such that the detector output does not substantially exceed thelevel set by the reference signal. In the preferred embodiment disclosedherein, this feedback loop provides linear negative feedback for thecontrol of the high voltage.

Stated somewhat differently, the voltage applied to the photomultiplierdetector is controlled by the adjustment signal as long as thephotomultiplier detector output remains below a predetermined fixedlimit below which the detector can be safely operated. However, once thephotomultiplier detector output reaches this limit, a negative feedbackloop then controls the voltage such that the photomultiplier detectoroutput does not rise substantially above the predetermined limit. Thus,it is seen that the photomultiplier detector is operated in an open loopfashion (i.e., controlled only by the adjustment signal) as long as thephotomultiplier detector output is within a safe operating range. Suchoperation is a necessary element of a single-beam spectrophotometer inthat the photomultiplier detector must be operated in an open-loopfashion during the measurement of the sample. However, once an excessiveoutput is detected because the incident light flux is excessive for thevoltage applied to the detector in response to the adjustment signal,the photomultiplier detector voltage is then controlled via a negativefeedback loop to protect the photomultiplier detector, that is, thedetector is then operated in a closed-loop manner. The closed-loopprotection system continues to control the voltage during the time thatan excessive incident light flux condition exists.

Once the excessive incident light flux condition ends, the closed-loopprotection system ceases to control the voltage applied to thephotomultiplier detector, and the voltage is again controlled by theadjustment signal in an open-loop fashion so that, for example,spectrophotometer operation can continue without further interruption.

BRIEF DESCRIPTION OF THE DRAWING

The sole FIGURE is a simplified schematic diagram of a photomultiplierdetector protection system in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the drawing, a photomultiplier detector protectionsystem 10 embodying the present invention includes a high voltage powersupply 12. The power supply 12 is of conventional design and can be, forexample, a switching-type power supply. The power supply 12 generates anegative output voltage that is controlled in accordance with an inputsignal derived as described hereinbelow. The output voltage is appliedto a photomultiplier detector shown generally as 14.

The photomultiplier detector 14 includes a photomultiplier tube 16 ofconventional design including a photo-emissive cathode 18, an optionalcontrol electrode 20, an electron multiplier section comprising aplurality of dynodes 22, and an anode 24. In a preferred embodiment, thephotomultiplier tube 16 is a commercially available type R928photomultiplier tube manufactured and sold by Hamamatsu TV Co. Ltd., asdescribed in a Hamamatsu catalog number SC-1-3 revised February, 1979.

The output from the high voltage power supply 12 is connected to thecathode 18 and is also applied to a voltage divider network 26 connectedto ground comprising a plurality of individual fixed resistors 28. Thevoltage divider network 26 divides the voltage from the power supply 12to provide increasing positive potentials to respective ones of thedynodes 22 from the cathode 18 to the anode 24. Light flux incident uponthe cathode 18 as shown by the arrows 30 produces electron flow from thecathode 18 by photo-emission. The electron flow is multiplied by thedynodes 22 and is ultimately collected by the anode 24. The outputcurrent from the anode 24 is applied across resistor 32 which is in turnconnected to ground and to a preamplifier 34, the output of whichprovides a preamplifier output signal. The preamplifier output isprocessed in a fashion well known in the art as, for example, in asingle-beam spectrophotometer. Thus it is seen that the power supply 12,the detector 14 and the preamplifier 34 are conventional elements of aphotomultiplier detector system.

The output from the preamplifier 34 is also connected through a resistor36 to a negative feedback comparator shown generally at 38. Thecomparator includes a conventional operational amplifier 40, theinverting input of which is connected to the resistor 36. The invertinginput is also connected via a conventional feedback compensation networkcomprising a capacitor 42 and a resistor 46 to the output of theamplifier 40.

Two resistors 48 and 50 form a voltage dividing network generallydesignated 52. One terminal of the resistor 48 is connected to asuitable voltage source (+V) while one terminal of the resistor 50 isconnected to ground. The common node between the resistors 48 and 50 isconnected to the non-inverting input of the amplifier 40. The network 52establishes a predetermined over energy limit corresponding to apredetermined safe maximum output current flow from the photomultiplierdetector 14 and a resulting preamplifier 34 output. The predeterminedsafe current flow is preferably somewhat less than the maximum currentpermissible through the photomultiplier detector 14 and establishes asafe operating range for the photomultiplier detector.

The output of the amplifier 40 is applied to the cathode of a diode 54,the anode of which is connected through a resistor 56 to a summing stagegenerally designated 58. The summing stage 58 comprises a conventionaloperational amplifier 60 having an inverting input connected to theresistor 56. The inverting input is also connected through aconventional feedback compensation network comprising a capacitor 62 anda resistor 63 connected to the output of the amplifier 60. A highvoltage set signal is applied to the non-inverting input of theamplifier 60. The high voltage set signal is generated by an open loopvoltage controller 61 to control the output voltage of the power supply12 and thus the sensitivity or gain of the photomultiplier detector 14.The open loop voltage controller 61 can be any conventional signalsource which is adjusted to control the sensitivity of the detector 14.In a single-beam spectrophotometer, for example, the open loop voltagecontroller 61 can comprise an adjustable digital counter 61a generatinga plurality of digital output signals that are applied to adigital-to-analog converter (DAC) 61b. The DAC 61b generates the highvoltage set signal according to the applied digital signals. The digitalcounter 61a output is adjusted to vary the high voltage set signalduring the dynode voltage adjustment done for the reference material adescribed hereinbefore and as is well known in the art.

Turning now to the operation of the photomultiplier detector protectionsystem 10, the high voltage set signal is amplified by the summing stage58 to generate a resulting high voltage adjustment signal that isapplied to the high voltage power supply 12. The power supply 12, inresponse to the magnitude of the adjustment signal, provides aproportional output voltage to the photomultiplier detector 14. Asincident light flux 30 is applied to the detector 14, current flowsthrough the detector 14 in proportion to the intensity of the light flux30 and the voltage applied from the power supply 12. The output isamplified by means of the preamplifier 34 and is applied to thecomparator 38.

The comparator 38 establishes the maximum output that is allowable fromthe preamplifier 34 and consequently the maximum safe output currentfrom the photomultiplier detector 14. In particular, when thecombination of incident light flux 30 and power supply 12 voltagegenerates a detector 14 output within the safe operating range, thepreamplifier output is below the over energy limit set by the voltagedividing network 52. Under such circumstances, the output of theamplifier 40 is positive with respect to the output of the summing stage58 and the diode 54 does not conduct, thereby isolating the summingstage 58 from the comparator 38.

If, however, the incident light flux 30 increases such that thecombination of incident light flux 30 and power supply 12 voltagegenerates a detector 14 output that is above the over energy limitestablished by the voltage dividing network 52, the output of thecomparator 38 from the amplifier 40 becomes negative with respect to theoutput of the amplifier 60. Consequently, the diode 54 conducts andapplies the output from the amplifier 40 through the diode 54 and theresistor 56 to the inverting input of the amplifier 60. This in turncauses a shift in the output of the amplifier 60 in a direction whichcauses a corresponding reduction (decrease in the magnitude of thenegative voltage toward zero) in the high voltage output from the powersupply 12. The reduced power supply voltage decreases the dynode voltageand the sensitivity or gain of the photomultiplier detector 14 andreduces the output of the preamplifier 34. As the magnitude of thecorrection to power supply voltage increases, as with further increasesin incident light flux 30, the magnitude of the negative output fromamplifier 40 correspondingly increases, thereby further reducing thepower supply voltage to maintain the detector 14 output substantiallyequal to the over energy limit set by the voltage divider network 52.

Assuming now that the incident light flux 30 decreases, the magnitude ofthe negative output of the amplifier 40 also decreases to consequentlyincrease the power supply voltage and thereby maintain the detector 14output substantially equal to the over energy limit set by the voltagedivider network 52. If the incident light flux 30 continues to decreasesuch that the combination of incident light flux 30 and power supplyvoltage established by the high voltage set signal results in a detectoroutput within the safe operating range, the detector 14 output becomesless than the over energy limit. Under such circumstances, thecomparator 38 is isolated from the summing stage 58 and the high voltageset signal again controls the power supply 12 output voltage.

In this way, the comparator 38 and summing amplifier 58 comprise anegative feedback loop which controls the output of the high voltagepower supply 12 to maintain the detector 14 output substantially at orbelow the limit established by the voltage dividing network 52. Once thelimit is reached, the comparator 38 provides linear negative feedback tothe summing amplifier 58 to correspondingly control the high voltagepower supply 12. Thus the photomultiplier detector protection system 10in accordance with the present invention provides continuous control ofthe detector 14 output. The power supply 12 is controlled in an openloop fashion by the high voltage set signal as long as the output fromthe detector 14 and hence the preamplifier 34 is less than the overenergy limit established by the voltage dividing network 52. However, ifthe detector output exceeds this limit, the high voltage power supply 12is then controlled in a closed-loop negative-feedback fashion to limitthe detector output. Consequently, the detector 14 is under constantcontrol, either open-loop or closed-loop.

It will be recognized that the comparator 38 and the summing stageoperate with equal effectiveness if the light flux 30 remains constantand the open loop voltage controller 61 generates a high voltage setsignal which would result in a detector 14 output current greater thanthe over energy limit. The negative feedback loop again controls thehigh voltage power supply 12 as described above to maintain the detector14 output current within a safe operating range.

Thus, the photomultiplier detector protection system of the presentinvention uniquely provides continuous control over the sensitivity ofthe photomultiplier detector to thereby prevent detector damage. Thisfeature allows the output of the detector to be used after andoccurrence of excessive incident light flux without having to reset orreadjust the control of the high voltage power supply. This isparticularly advantageous in a single-beam spectrophotometer wherecontinued spectrophotometer operation is desired after such anoccurrence without resetting the control of the high voltage powersupply.

While a preferred embodiment of the present invention has beenillustrated and described, it will be understood that variousmodifications may be made therein without departing from the subject andscope of the appended claims.

What is claimed is:
 1. An improved photomultiplier detector protectionsystem wherein an adjustment signal from external means adjusts thesensitivity of the photomultiplier detector, the system comprising:apower supply for generating a variable output voltage in response to acontrol signal; a photomultiplier detector responsive to incident lightflux and to the power supply output voltage for generating an outputrelated to the incident light flux and the magnitude of the power supplyoutput voltage; reference means for generating a reference signalproportional to a predetermined photomultiplier detector output;comparator means responsive to the reference signal and responsive tothe photomultiplier detector output for comparing the reference signaland the photomultiplier detector output to generate an output when thereference signal and the photomultiplier detector output are in apredetermined relationship; means for receiving the adjustment signal;and summing means responsive to the adjustment signal from the receivingmeans and to the comparator means output for generating the controlsignal by summing the adjustment signal and the comparator means output.2. A system as in claim 1 wherein the comparator means includes meansfor generating the comparator means output when the photomultiplierdetector output is greater than the reference signal.
 3. A system as inclaim 2 wherein the comparator means includes means for selecting thepolarity of the comparator means output to decrease the photomultiplierdetector output.
 4. A system as in claim 3 wherein the comparatorincludes means for generating the comparator output linearly related tothe magnitude of the difference between the reference signal and thephotomultiplier detector output.
 5. An improved photomultiplier detectorprotection system wherein an adjustment signal from external meansadjusts the sensitivity of the photomultiplier detector, the systemcomprising:a power supply for generating a variable output voltage inresponse to a control signal; a photomultiplier detector responsive toincident light flux and to the power supply output voltage forgenerating an output related to the incident light flux and themagnitude of the power supply output voltage; reference means forgenerating a reference signal proportional to a predeterminedphotomultiplier detector output; comparator means responsive to thereference signal and responsive to the photomultiplier detector outputfor comparing the reference signal and the photomultiplier detectoroutput to generate an output related to the magnitude of the differencebetween the reference signal and the photomultiplier detector outputwhen the photomultiplier detector output is greater than the referencesignal; means for receiving the adjustment signal; summing meansresponsive to the adjustment signal from the receiving means and to thecomparator means output for generating the control signal by summing theadjustment and the comparator means output; and the comparator meansincludes means for selecting the polarity of the comparator means outputto decrease the photomultiplier detector output.
 6. A method ofprotecting a photomultiplier detector wherein an adjustment signal fromexternal means adjusts the sensitivity of the photomultiplier detector,the method comprising:generating a variable voltage in response to acontrol signal; generating a photomultiplier detector output related tothe light flux incident upon the photomultiplier detector and themagnitude of the variable voltage; generating a reference signalproportional to a predetermined photomultiplier detector output;comparing the reference signal and the photomultiplier detector output;generating a negative feedback signal when the reference signal and thephotomultiplier detector output are in a predetermined relationship;receiving the adjustment signal; and generating the control signal bysumming the adjustment signal and the negative feedback signal.
 7. Amethod as in claim 6 wherein the step of generating a negative feedbacksignal includes generating the negative feedback signal when thephotomultiplier detector output is greater than the predeterminedphotomultiplier detector output represented by the reference signal. 8.A method as in claim 7 including the step of selecting the polarity ofthe negative feedback signal to decrease the photomultiplier detectoroutput.
 9. A method as in claim 8 wherein the step of generating anegative feedback signal includes generating a negative feedback signallinearly related to the magnitude of the difference between thereference signal and the photomultiplier detector output.
 10. A methodof protecting a photomultiplier detector wherein an adjustment signalfrom external means adjusts the sensitivity of the photomultiplierdetector, the method comprising:generating a variable voltage inresponse to a control signal; generating a photomultiplier detectoroutput related to the light flux incident upon the photomultiplierdetector and the magnitude of the variable voltage; generating areference signal proportional to a predetermined photomultiplierdetector output; comparing the reference signal and the photomultiplierdetector output; generating a negative feedback signal with a polarityselected to decrease the photomultiplier detector output when thephotomultiplier detector output is greater than the predeterminedphotomultiplier detector output represented by the reference signal;receiving the adjustment signal; and generating the control signal bysumming the adjustment signal and the negative feedback signal.